In the early days of railroads, train brakes were operated by brakemen who would go from car to car and manually activate and deactivate the brakes on the train. Having brakemen on each train added to the expense of operating the train. This added expense led to the development of air brakes by, among others, George Westinghouse.
For much of the previous century, the most common form of braking system was the air brake system. In this system, pressurized air is distributed via an air brake pipe system to each brake caliper on each train. The brake calipers are designed such that the brake shoes engage the train wheel to stop the train if the pressurized air flow is disrupted. These systems typically include what is referred to in the art as a “P2A” valve, which is used for a “penalty” braking. Penalty braking, which is distinct from emergency braking, is the activation of the train's brakes to stop the train when the train is operating, or about to be operated, in an unsafe manner. The typical P2A valve is connected to the brake pipe and typically provides for a full service application of the brakes at the service rate when opened. The P2A valve is electrically controlled, usually employing a solenoid. This allows the P2A valve to be controlled by an overspeed signal from a speed indicator connected to the train's axle drive tachometer, by a penalty brake signal from a cab signal system, or by an alerter. These air brake systems that include a P2A valve are failsafe in that any loss of air pressure in the brake lines (whether due to a leak in the brake line or a failure in the air pump), or any disruption in power to the P2A valve, results in the brakes activating and the train being brought to a stop safely.
More recently, electronic braking systems have appeared. These systems electronically control the application of the brakes. These systems are required to be failsafe by the federal government in that any loss of power to the electronic braking system must result in the train brakes activating to stop the train much in the same way that any loss of power to the P2A valves would result in activation of the train brakes.
In addition to electronic braking systems, train control systems are also known in the art. Train control systems are systems that control the movement of a train by controlling the locomotive's engine/motor and brakes to ensure that the train is operated safely. These systems come in two varieties: active and passive. Active control systems are systems that are primarily responsible for controlling movement of the train. In contrast, in passive control systems, a human operator is primarily responsible for controlling movement of the train and the passive control system only assumes control if the operator attempts to operate the train in an unsafe manner (e.g., exceeding a maximum allowable speed, entering an occupied block, etc.). Types of train control systems include Cab Signal, Positive Train Control, Positive Train Stop and others. The assignee of the present application, Quantum Engineering, Inc., markets a Positive Train Control system under the mark TRAIN SENTINEL™.
In order for a train control system of any type to be capable of stopping a train, it must be capable of controlling the train's braking system. These electronic braking systems are typically integrated, sealed units that are not easily modified. Thus, in the past, it has typically been necessary to enlist the assistance of the manufacturer of the electronic braking system to modify the electronic braking system to allow for a penalty application of the brakes by a train control system.
Typical electronic braking systems provide an RS-232 interface through which a train control system can send a request to activate the brakes. However, such a system is not failsafe. For example, if the connection between the train control system and the RS-232 interface is broken, or the RS-232 interface on the electronic braking system fails, a brake activation request message from the train control system to the electronic braking system will not be received by the electronic braking system and the brakes will not be activated, leading to a potentially dangerous situation.
Moreover, even if the brake activation request message is received, the electronic braking system may experience a delay in responding to the message. Any delay can, of course, result in a catastrophe. There is yet another problem associated with a potential delay. Some known train control systems, including the Train Sentinel system sold by the assignee of the present invention, will prevent a train from taking an improper action (e.g., entering a block which the train is not authorized to enter) by calculating in advance precisely when a penalty brake application must be applied to avoid the improper action, and then causing the penalty brake application to occur at that time if the operator has not acted to avoid the improper action. The possibility that an electronic braking system will delay in activating the brakes in response to a penalty brake activation request message means that the penalty brake activation request message must be sent in advance of the time in which the brakes need to be activated—in other words, a safety factor must be added to account for the delay must be provided. There are two problems with such a safety factor: 1) it is difficult to determine how large the safety factor must be, and 2) in those instances in which there is no delay, or in which the delay is less than the safety factor, the brakes activate prematurely. Premature penalty brake activation can be very annoying to the operator and can be costly because the train will stop well short of the intended point and because of the lost time required to stop and restart the train.